Generators for use in high output, high speed applications are devices with critical weight and volume restrictions. Such devices typically have permanent magnet rotors, which have a plurality of rare earth permanent magnet elements located on a shaft of magnetized material. The shaft has a number of flat machined faces about its periphery, onto which the rare earth permanent magnet elements are mounted.
Generally, spacers made of aluminum or some other non-magnetizable material are located around the periphery of the shaft in the area between the magnets, and rings of the same type of non-magnetizable material are located on both ends of the shaft axially adjacent the annulus of permanent magnets and spacers. An outer sleeve made of non-magnetizable material is shrink-fitted over the magnets, spacers, and rings to prevent the magnets from moving radially outward during high speed rotation of the rotor.
While such construction provides acceptable characteristics for most generator applications, it also presents several major problems in state-of-the art generators operating at extremely high speed and providing very high output, resulting in reduced performance, high reject rate, and a relatively high unit cost. One significant problem is that of heat build up in the rotor. The heat transfer characteristics of rotors constructed of spacers and rings as described above are not uniform, and may thus lead to heat buildup possibly resulting in damage to the rare earth permanent magnets. In addition, it is apparent that machining the spacers and rings to fit together properly with the magnets is fairly difficult and requires extremely close tolerances. If the rings and spacers in the rotors do not fit exactly, the rotor will not be stable at very high rotational speeds, thus allowing relative movement of the components under the outer sleeve and an unbalanced condition in the rotor which may result in dynamic failure of the device.
A more serious problem encountered in the construction of such a permanent magnet rotor is that the rotor assembly does not have good rigidity or good shaft stiffness, thus reducing the flexure critical speed of the device, which is the maximum speed of rotation without significant dynamic vibration occuring. Since the rotor must be designed to operate at a very high rate of speed, the lack of proper stiffness in rotor construction will result in a high rejection rate at best, and possibly in a product which will not perform within the required specifications. It may be appreciated that manufacture of a permanent magnet rotor with a shaft, magnets, spacers and rings, and the outer sleeve is a very expensive method of construction. The very precise tolerance requirements of the spacers and rings and the high unit rejection rate both add further to the high cost of construction to such rotors.
An alternative rotor construction resulting in a substantially improved rotor capable of higher output, higher speed operation is described in U.S. patent application No. 515,331, now U.S. Pat. No. 4,549,341, entitled "Permanent Magnet Rotor and Method for Producing Same" filed July 19, 1983 by George Kasabian, which application is assigned to the assignee of the present invention, and is hereby incorporated by reference herein. This construction involves making a rotor by casting a non-magnetic material, preferably aluminum, directly onto the steel shaft with pockets or apertures for the permanent magnets cast into the aluminum, thereby reducing the need for machining. Following a small amount of machining of the assembly containing the magnets, a non-magnetic outer sleeve is then heat shrunk onto the shaft.
This construction results in a completed rotor assembly having substantially improved rigidity characteristics, with an accordingly increased flexure critical speed and reduced possibility of dynamic unbalance in the rotor leading to machine failure. Since rings and spacers are not used in this technique, the tolerance problems accompanying this use is minimized. Additionally, since a reduced amount of machining is necessary the rotor is more economical to manufacture.
While this manufacturing technique increases rotor rigidity, it is desireable to have an even more rigid rotor to allow construction of a longer rotor. Longer rotors are desirable since length of the rotor is generally proportional to output capacity of the resulting machine; however, with longer rotors of similar diameter and characteristics flexure critical speeds tend to decrease. In order to obtain both maximum output and maximum operating speed (which aids in producing maximum output), it is apparent that it is necessay to further increase rigidity of the rotor over the aluminum molding technique.
While increasing the rigidity of the rotor, it is necessary to retain the other advantages of the aluminum injected rotor. For example, machining of parts should be kept to a minimum to reduce overall cost per unit. The number of parts should also be kept to a minimum, thereby avoiding the main design problem of spacer and ring construction, improper fit, while still minimizing heat buildup by ensuring a tight fit between the rotor and the magnets. Finally, the resulting electrical machine must provide both high output and high speed operation in a minimal size package and at an economically feasible cost while having a low unit rejection rate.